Regular travel Brussels to Aix en Provence

You could look at the Carte Avantage, which gives you discounts on TGV, including the international TGV from Brussels.

https://www.sncf-connect.com/en-en/catalogue/description/carte-avantage-adulte

Assuming you are resident in Belgium, you could also use an Interrail pass within France - eg from Lille to Aix - paying as little as €10 each way for a reservation on the TGV or Ouigo.

You’ll be able to use this pass on two days within Belgium, but for the rest of your trips you’ll need some kind of pass or ticket within Belgium and to reach Lille, or at least the French border.

Within Belgium, for regular journeys, a Train+ card usually makes sense, since it normally amortises very quicky. If you're a student, then also look into the Student Multi.

For the TGV Brussel-France. there's also a "Pass 2” fare (info in nl/fr/en), which is valid with a pass only valid in France. Until recently, you could see that the price was exactly the same a pass reservation for a global pass, but unfortunately, Belgian railways don't public the prices anymore and you can't book it online. In any case, you could ask Belgian railways (e.g. on Facebook chat) whether that's still the case, because then it may be cheaper to buy a one country pass just for France. I'd be inclined then to buy regular pass reservations online.